Abstract

A special proteolipid protein fraction having a high affinity for binding to acetylcholine and other cholinergic drugs was isolated from electroplax of Electrophorus electricus. After extraction in chloroform-methanol (2:1 by volume) and column chromatography on Sephadex LH-20, the proteolipid appeared in a special peak together with the radioactive acetylcholine and increased in proportion with the amount of protein applied to the column. A saturation curve of this peak with respect to concentrations of 14C-acetylcholine between 7 x 10-7 and 5 x 10-5 M was constructed. The shape of this curve and the double-reciprocal plot of the data suggested the presence of more than one type of binding site. Assuming a homogeneous population of molecules having a molecular weight of 40,000, the use of the Scatchard equation suggested that there is a single high-affinity binding site with an apparent dissociation constant of 1 x 10-7 M and a group of low-affinity sites with a dissociation constant of 1 x 10-5 M. Since this proteolipid is present only in the electroplax membranes, these results are discussed in relation to the possible quantity of this material per electroplax or synaptic junction and its possible physiological significance as a receptor in cholinergic synapses.

ACKNOWLEDGMENTS The authors are indebted to Professor Gregorio Weber of the University of Illinois, to Dr. Enrique Rosengurt of the Instituto de Investigaciones Bioquímicas, and Dr. George G. Lunt for their helpful suggestions in the analysis of the data.